Polyphosphoric acid

Polyphosphoric acid (PPA) is a strong mineral acid that combines orthophosphoric acid and linear phosphoric acids. It is widely used in organic synthesis because of its powerful dehydrating properties, low nucleophilicity of the phosphoric acid media, good solvation power, relatively mild acidity, and low oxidation potential. Even though PPA is a significantly weaker acid than H₂SO₄, it demonstrates 100% dehydration rate.

Polyphosphoric acid (PPA) can be used as a reagent:

• In acylation and alkylation reactions.
• To synthesize cyclic aromatic compounds via cyclization of aromatic acids, esters, ketones, aldehydes, acetals, alcohols, and alkenes.
• In Bischler−Napieralski reaction to construct isoquinoline ring system from phenethylformamides.
• To prepare substituted indoles at position-2 from hydrazones via Fisher indole synthesis.
• For the conversion of oximes into amides via Beckmann rearrangement.

PPA can also be used as a catalyst to synthesize:

• Aniline derivatives by Lossen rearrangement of aromatic carboxylic acids with nitromethane.
• Amides by hydrolysis nitriles.
• Dimethyl carbonate (DMC) from urea and methanol.
• Benzofurans by cyclization of α-phenoxy ketones.

PPA is also used with other reagents for various organic transformations. For example:

• PPA/Ethanethiol mixture can be used to selectively open one epoxide in bis-epoxy steroid.
• PPA/POCl₃ catalytic mixture used to convert tertiary alcohols into corresponding chlorides.
• PPA/acetic acid is used to synthesize nonenolizable β-diketones.
• PPA/potassium iodide mixture used to convert primary methyl ether to a primary alkyl iodide.
• PPA/nitric acid catalytic combination used for nitration reactions which is less hazardous than Ac₂O/HNO₃ mixtures.

Assay (acidimetric, calc. as P₂O₅): 83.0 - 87.0 %
Due to its specific melting range the product may be solid, liquid, a solidified melt or a supercooled melt.